Semiconductor device manufactured with a double shallow trench isolation process

ABSTRACT

A method for manufacturing a semiconductor device includes forming a device isolation film by a double Shallow Trench Isolation (STI) process, forming a first active region having a negative slope and a second active region having a positive slope. Additionally, the method includes applying a recess region and a bulb-type recess region to the above-extended active region so as to prevent generation of horns in the active regions. This structure results in improvement in effective channel length and area.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Korean patent application number 10-2006-0060088, filed on Jun. 30, 2006, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to a semiconductor device and a method for manufacturing the same, and more specifically, to a technology of forming a device isolation film by a double Shallow Trench Isolation (STI) process, forming a first active region having a negative slope and a second active region having a positive slope and applying a recess region and a bulb-type recess region to the above-extended active region so as to prevent generation of horns in the active regions and to improve a channel length and area effectively.

As a result of high integration of semiconductor devices, a process margin for forming an active region and a device isolation film has been reduced. As a line width of a gate has become narrower, the channel length has also been reduced to degrade electric characteristics of semiconductor devices and generate a short channel effect. In order to prevent the degradation of semiconductor devices and the generation of the short channel effect, a recess gate obtained by etching a given depth of a semiconductor substrate of a local gate region has been used to increase the contact area between the active region and the gate and also increase the gate channel length.

SUMMARY OF THE INVENTION

Various embodiments are directed at providing a semiconductor device and a method for manufacturing the same that includes forming a device isolation film by a double STI process, forming a first active region having a negative slope and a second active region having a positive slope and applying a recess region and a bulb-type recess region to the above-extended active region so as to prevent generation of horns in the active regions and to improve a channel length and an area effectively.

According to an embodiment of the present invention, a semiconductor device comprises a first trench having a negative slope over a semiconductor substrate, a second trench obtained by etching the lower part of the first trench to have a smaller width than that of the first trench and have a positive slope, a device isolation film for filling the first and second trenches, an active region defined by the device isolation film, and a gate formed over the active region.

The gate includes a recess gate whose recess region overlapped with the active region is etched. The recess region has the active region whose center defined by the second trench is etched at a given depth, that is, whose cross-section taken along a gate direction is formed to have a ‘U’ shape along a line width of the active region defined by the first trench.

The gate includes a bulb-type recess gate obtained by isotropic-etching a first recess region formed in the active region overlapped with the gate and the lower part of the first recess region. The first recess region has the active region whose center defined by the second trench is etched at a given depth, that is, whose cross-section taken along a gate direction is formed to have a ‘U’ shape along a line width of the active region defined by the first trench.

According to an embodiment of the present invention, a method for manufacturing a semiconductor device comprises the steps of: forming a first trench having a negative slope that defines an active region over a semiconductor substrate and whose sidewall gets wider downward; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench with the hard mask barrier layer as an etching barrier wall to form a second trench having a positive slope whose sidewall becomes narrower; removing the hard mask barrier layer; forming a device isolation film for filling the first and second trenches; and forming a gate over the semiconductor substrate.

The first trench has a depth ranging from about 1,000 to about 5,000 Å. The hard mask barrier layer is formed with one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å. The second trench has a depth ranging from about 1,000 to about 5,000 Å. A sidewall barrier layer is further formed at sidewalls of the first and second trenches. The sidewall barrier layer is formed with one selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å.

According to an embodiment of the present invention, a method for manufacturing a semiconductor device comprises the steps of: forming a first trench having a negative slope that defines an active region over a semiconductor substrate and whose sidewall gets wider downward; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench with the hard mask barrier layer as an etching barrier wall to form a second trench having a positive slope whose sidewall becomes narrower; removing the hard mask barrier layer; forming a device isolation film for filling the first and second trenches; etching active regions defined by the device isolation film and overlapped with a subsequent gate to form a recess region; and forming a gate over the semiconductor substrate including the recess region.

The first trench has a depth ranging from about 1,000 to about 5,000 Å. The hard mask barrier layer is formed with one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å. The second trench has a depth ranging from about 1,000 to about 5,000 Å. A sidewall barrier layer is further formed at sidewalls of the first and second trenches. The sidewall barrier layer is formed with one selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å. The recess region has a depth ranging from about 1,000 to about 5,000 Å.

The recess region has the active region whose center defined by the second trench is etched at a given depth, that is, whose cross-section taken along a gate direction is formed to have a ‘U’ shape along a line width of the active region defined by the first trench. The active region defined by the second trench is etched so that the height of the ‘U’-shaped portion ranges from about 100 to about 2,000 Å.

According to an embodiment of the present invention, a method for manufacturing a semiconductor device comprises the steps of: forming a first trench having a negative slope that defines an active region over a semiconductor substrate and whose sidewall gets wider downward; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench with the hard mask barrier layer as an etching barrier wall to form a second trench having a positive slope whose sidewall becomes narrower; removing the hard mask barrier layer; forming a device isolation film for filling the first and second trenches; etching active regions defined by the device isolation film and overlapped with a subsequent gate to form a first recess region; isotropic-etching the lower part of the first recess region to form a second recess region and a bulb-type recess region including the first and second recess regions; and forming a gate over the semiconductor substrate including the bulb-type recess region.

The first trench has a depth ranging from about 1,000 to about 5,000 Å. The hard mask barrier layer is formed with one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å. The second trench has a depth ranging from about 1,000 to about 5,000 Å. A sidewall barrier layer is further formed at sidewalls of the first and second trenches. The sidewall barrier layer is formed with one selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å. The first recess region has a depth ranging from about 1,000 to about 5,000 Å. The second recess region has a depth and line width ranging from about 100 to about 2,000 Å.

The first recess region has the active region whose center defined by the second trench is etched at a given depth, that is, whose cross-section taken along a gate direction is formed to have a ‘U’ shape along a line width of the active region defined by the first trench. The active region defined by the second trench is etched so that the height of the ‘U’-shaped portion ranges from about 100 to about 2,000 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plane diagram illustrating a conventional semiconductor device.

FIGS. 2 and 3 are cross-sectional diagrams illustrating a conventional semiconductor device.

FIG. 4 is a plane diagram illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 5( i) and 5(ii) are cross-sectional diagrams illustrating a semiconductor device and a method for manufacturing the same according to an embodiment of the present invention.

FIG. 6 is a plane diagram illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 7( i) and 7(ii) are cross-sectional diagrams illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 8( i) and 8(ii) are cross-sectional diagrams illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 9( i) and 9(ii) are cross-sectional diagrams illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 10( i) and 10(ii) are cross-sectional diagrams illustrating a semiconductor device according to an embodiment of the present invention.

FIGS. 11 a through 11 f are cross-sectional diagrams illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

FIGS. 12( i) and 12(ii) are cross-sectional diagrams illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

FIGS. 13( i) and 13(ii) are cross-sectional diagrams illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

FIGS. 14( i) and 14(ii) are cross-sectional diagrams illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a plane diagram illustrating a conventional semiconductor device.

An active region 20 and a device isolation film 30 are formed over a semiconductor substrate 10, and a recess region 40 obtained by etching the active region 20 of a local gate region is formed. A gate 50 is formed for filling the recess region 40.

FIGS. 2 and 3 are cross-sectional diagrams illustrating a conventional semiconductor device.

FIG. 2 shows a cross-section taken along a direction XX′. The gate is formed over the recess region 40, and a channel length is shown to increase by the recess region 40 in the length direction of the gate 50.

However, there is a limit in increasing of the channel length with the recess region as semiconductor devices become smaller. The channel area determined by the active region has been reduced so as to decrease operation current. As a result, the electric characteristics of semiconductor devices are degraded.

FIG. 3 shows a cross-section taken along a direction YY′. As the line width of the active region 20 has been reduced, a process margin for etching the recess region 40 is limited. The active region 20 is not etched in the lower part of the recess region 40 so that the active region 20 remains as a horn-shape to generate a horn 60. The horn 60 is generated by defects in a subsequent process to degrade the electric characteristics of semiconductor devices.

As mentioned above, the process margin for forming a device isolation film and an active region in a high-integrated semiconductor device has been reduced, and a recess region formed to improve an electric characteristic of a gate serves as a factor which generates defects of the semiconductor device.

FIG. 4 is a plane diagram illustrating a semiconductor device according to an embodiment of the present invention.

A first bar-type active region 120 a defined by a first trench is formed over a semiconductor substrate 100. The lower part of the first trench is etched to form a second active region 120 b defined by a second trench having a given width narrower than that of the first trench. A device isolation film 130 is formed for filling the first and second trenches. As the second trench becomes smaller, the second active region 120 b whose area becomes larger than that of the first active region 120 a is formed in the bottom of the first active region 120 a. Although the second active region 120 b is not shown in FIG. 4, FIG. 4 shows the area increment of the first active region 120 a. A gate 200 is formed perpendicular to the length direction of the active region 120.

FIG. 5 is a cross-sectional diagram illustrating a semiconductor device and a method for manufacturing the same according to an embodiment of the present invention. (i) shows a cross-section taken along a direction XX′of FIG. 4 and (ii) shows a cross-section taken along a direction YY′ of FIG. 4.

A first trench 130 a that defines a bar-type active region 120 is formed over a semiconductor substrate 100 to have a negative slope whose sidewall becomes wider downward based on a device separating region. The first trench 130 a is formed to have a depth ranging from about 1,000 to about 5,000 Å by a Shallow Trench Isolation (STI) process.

A hard mask barrier layer (not shown) is formed over the semiconductor substrate 100. The low part of the first trench 130 a is etched with the hard mask barrier layer to form a second trench 130 b having a positive slope whose sidewall becomes narrower downward. The hard mask barrier layer is formed with one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å. Since the low part of the first trench is etched with the hard mask barrier layer as an etching sidewall, the second trench 130 b becomes smaller by 10-500 Å than the first trench so that the active region becomes larger relatively. The second trench 130 b is formed to have a depth ranging from about 1,000 to about 5,000 Å by a STI process. In this way, the device separating trenches are formed by the double STI process to secure an etching margin and increase the area of the active region and the channel area, thereby improving the current driving capacity.

The hard mask barrier layer is removed to fill a device isolation film 130 in the first and second trenches 130 a and 130 b. The device isolation film 130 includes a High Density Plasma (HDP) oxide film. In order to protect sidewalls of the trenches, a sidewall barrier layer (not shown) is further formed at sidewalls of the first and second trenches 130 a and 130 b to fill the device isolation film 130. The sidewall barrier layer is formed with one selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200Å.

A gate 200 is formed over the semiconductor substrate 100. Specifically, a gate oxide film, a gate polysilicon layer, a gate metal layer and a hard mask layer are sequentially deposited over the semiconductor substrate 100. An etching process is performed on the deposition structure with a gate mask to obtain the gate 200.

FIG. 6 is a plane diagram illustrating a semiconductor device according to an embodiment of the present invention.

The semiconductor device of FIG. 6 is formed to have the same structure as that of FIG. 4 so that the semiconductor device comprises an extended active region 120. The active region 120 whose portion is overlapped with a gate 200 is etched to form a recess region 150 for increasing a channel length of the gate 200. The recess region 150 is formed perpendicular to the uni-axial direction of the first active region 120 a to have a narrower line-width than that of the gate 200.

FIG. 7 is a cross-sectional diagram illustrating a semiconductor device according to an embodiment of the present invention. (i) shows a cross-section taken along a direction XX′ of FIG. 6 and (ii) shows a cross-section taken along a direction YY′ of FIG. 6.

The double STI process is performed, as shown in FIG. 5, to form the active region 120 having a stair-shaped sidewall. The active region of the local gate region is etched to form the recess region 150. The gate 200 is formed for filling the recess region 150. since the top of the active region 120 having the recess region 150 has a negative slope, no horns are formed in the bottom of the recess region 150.

FIG. 8 is a cross-sectional diagram illustrating a semiconductor device according to an embodiment of the present invention.

A recess region 160 is formed to have the same structure as that of FIG. 7. Along a line width of a first active region 120 a defined by a first trench 130 a, a second active region 120 b defined by a second trench 130 b is further etched at a given depth. That is, the active region 120 including the recess region 160 in the gate direction has a cross-section cut in the gate direction to have a ‘U’ shape (see (ii) of FIG. 8).

FIG. 9 is a cross-sectional diagram illustrating a semiconductor device according to an embodiment of the present invention.

A bulb-type recess region 170 is formed over an active region 120 by a double STI process. The increase of the active region by the double STI process increases the channel area, and the bulb-type recess region 170 increases the channel length, thereby improving electric characteristics of the gate effectively.

FIG. 10 is a cross-sectional diagram illustrating a semiconductor device according to an embodiment of the present invention.

(i) and (ii) of FIG. 10 are combinations of FIG. 8 and FIG. 9. As shown in FIG. 8, the bulb-type recess region 180 is formed to be longer so that the active region 120 including the bulb-type recess region 180 is formed to have a ‘U’ shape.

FIGS. 11 a through 11 f are cross-sectional diagrams illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention. (i) shows a cross-section taken along a direction XX′ of FIG. 6 and (ii) shows a cross-section taken along a direction YY′ of FIG. 6.

A hard mask pattern 110 that defines the first bar-type active region 120 a is formed over the semiconductor substrate 100. The hard mask pattern 110 has a deposition structure including a pad oxide film and a pad nitride film. The semiconductor substrate 100 is etched at a given depth with the hard mask pattern 110 as an etching sidewall to form the first trench 130 a that defines the first active region 120 a. The first trench 130 a is formed at a depth ranging from about 1,000 to about 5,000 Å by a STI process to have a negative slope whose sidewall of the first trench 130 a becomes wider.

The hard mask barrier layer 140 is formed over the semiconductor substrate. The hard mask barrier layer 140 is formed with one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å.

An etch-back process is performed so that the hard mask barrier layer 140 remains at sidewalls of the hard mask pattern 110 and the first trench 130 a. The low part of the first trench 130 a is etched with the hard mask pattern 110 and the hard mask barrier layer 140 as etching barriers to form the second trench 130 b that defines the second active region 120 b. The second trench 130 b is formed at a depth ranging from about 1,000 to about 5,000 Å by a STI process to have a positive slope whose sidewall becomes narrower. The double STI process is performed to form the device separating trenches to secure the etching margin of the trenches. The sidewalls of the first and second active regions 120 a and 120 b are formed to have a stair-shaped step difference. As a result, the active region 120 is formed to increase the channel area corresponding to the step difference portion, increase the operating current and improve the current driving capacity.

The hard mask barrier layer 140 and the hard mask pattern 110 are removed, and the device isolation film 130 is filled in the first and second trenches 130 a and 130 b. The device isolation film 130 includes a HDP oxide film. In order to protect the sidewalls of the trenches, a sidewall barrier layer (not shown) is further formed at the sidewalls of the first and second trenches 130 a and 130 b to fill the device isolation film 130. The side barrier layer is formed with one selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å.

A region where a subsequent gate is overlapped with the first active region 120 a is etched to form the recess region 150 that increases the channel length. The active region 120 overlapped with the gate is etched at a depth ranging from about 1,000 to about 5,000 Å, and the recess region 150 is formed to have a given width narrower than that of the gate. The recess region 150 which contacts with the device isolation film 130 has a bottom whose area becomes smaller. The cross-section of the active region 120 is formed to have a stair-shaped step difference. The stair-shaped step difference formed over the active region 120 is rounded to have a saddle type. As a result, there are no horns in the lower part of the recess region 150.

A gate oxide film (not shown) is formed over the recess region 150, and a gate polysilicon layer (not shown) is formed to fill the recess region 150. The gate polysilicon layer is planarized. A gate metal layer and a gate hard mask layer are sequentially deposited over the gate polysilicon layer. An etching process is performed to etch the deposition structure with a gate mask, thereby obtaining the gate 200.

FIG. 12 is a cross-sectional diagram illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

By the same processes as shown in FIGS. 11 a through 11 e, the recess region 160 is formed. The second active region 120 b is further etched at a given depth along the lower line width of the first active region 120 a defined by the first trench 130 a. That is, the active region 120 including the recess region 160 has a cross-section cut in the gate direction to have a ‘U’ shape (see (ii) of FIG. 12). The height of the ‘U’ shape portion (a) ranges from about 100 to about 2,000 Å.

FIG. 13 is a cross-sectional diagram illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

The recess region 150 of FIG. 11 e corresponds to a first recess region 155. The low part of the first recess region 155 is isotropic-etched to form a second recess region 165. As a result, a bulb-type recess region 170 including the first and second recess region 155 and 165 is formed. The first recess region1 55 has a depth ranging from about 1,000 to about 5,000 Å, and the second recess region 165 has a depth and line width ranging from about 100 to about 2,000 Å. The bulb-type recess region 170 increases the channel length, thereby improving electric characteristics of the gate effectively.

FIG. 14 is a cross-sectional diagram illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

A bulb-type recess region 180 is obtained by combining the examples of FIGS. 12 and 13. The cross-section of the active region 120 is formed to have a ‘U’ shape by the second recess region 165 which becomes a bulb portion of the bulb-type recess region The second recess region has a depth and line width ranging from about 100 to about 2,000 Å, and the height of the ‘U’ shape portion ranges from about 100 to about 2,000 Å.

As described above, in a method for manufacturing a semiconductor device according to an embodiment of the present invention, a double STI process is performed to form a first trench having a negative slope, a first active region defined by the first trench, a second trench having a positive slope and a second active region defined by the second trench. The area of the second active region is increased to increase the operating current of a gate and improve gate characteristics. In order to increase the channel length, a local gate region of the first and second active regions is etched to form a recess region so that there are no horns in the recess region by the first trench having a negative slope. The recess region may be formed only in the first active region or may have a lower active region to have a ‘U’ shape. The recess region is formed to have a bulb-type, thereby increasing the gate channel length effectively.

Also, the double STI process is performed to form a device isolation film, thereby increasing the process margin. The sidewall of the active region is formed to have a stair shape, thereby increasing the bottom area of the active region to increase the channel area. When the recess region and the bulb-type recess region are applied in the above extended active region, there are no horns by the active region having a negative slope to increase the channel length and area effectively. Therefore, the method increases the process margin of semiconductor devices and electric characteristics of semiconductor devices.

The above embodiments of the present invention are illustrative and not limitative. Various alternatives and equivalents are possible. The invention, is not limited by the type of deposition, etching polishing, and patterning steps described herein. Nor is the invention limited to any specific type of semiconductor devices. For example, the present invention may be implemented in a dynamic random access memory (DRAM) device or non volatile memory device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims. 

1-7. (canceled)
 8. A method for manufacturing a semiconductor device, the method comprising the steps of: forming a first trench having a negative slope in a semiconductor substrate to defines an active region; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench by using the hard mask barrier layer as an etching mask to form a second trench having a positive slope; removing the hard mask barrier layer; forming a device isolation film to fill the first and second trenches; and forming a gate over the semiconductor substrate.
 9. The method according to claim 8, wherein the sidewalls of the first trench formed with the negative slope become wider downward based on a device separating region.
 10. The semiconductor device according to claim 8, wherein the sidewalls of the second trench formed with the positive slope become narrower downward.
 11. The method according to claim 8, wherein the first trench has a depth ranging from about 1,000 to about 5,000 Å.
 12. The method according to claim 8, wherein the hard mask barrier layer is one selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å.
 13. The method according to claim 8, wherein the second trench has a depth ranging from about 1,000 to about 5,000 Å.
 14. The method according to claim 8, further comprising a sidewall barrier layer on sidewalls of the first and second trenches.
 15. The method according to claim 14, wherein the sidewall barrier layer is selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200Å.
 16. A method for manufacturing a semiconductor device, the method comprising the steps of: forming a first trench having a negative slope in a semiconductor substrate to define an active; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench by using the hard mask barrier layer as an etching mask to form a second trench having a positive slope; removing the hard mask barrier layer; forming a device isolation film to fill the first and second trenches; etching a given region of the active region to form a recess region; and forming a gate over the semiconductor substrate including the recess region.
 17. The method according to claim 16, wherein the sidewalls of the first trench formed with the negative slope become wider downward based on a device separating region.
 18. The method according to claim 16, wherein the sidewalls of the second trench formed with the positive slope become narrower downward.
 19. The method according to claim 16, wherein the first trench has a depth ranging from about 1,000 to about 5,000 Å.
 20. The method according to claim 16, wherein the hard mask barrier layer is selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å.
 21. The method according to claim 16, wherein the second trench has a depth ranging from about 1,000 to about 5,000 Å.
 22. The method according to claim 16, further comprising forming a sidewall barrier layer on sidewalls of the first and second trenches.
 23. The method according to claim 22, wherein the sidewall barrier layer is selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å.
 24. The method according to claim 16, wherein the recess region has a depth ranging from about 1,000 to about 5,000 Å.
 25. The method according to claim 16, wherein the recess region is disposed in the semiconductor substrate under the gate, wherein the lower part of the recess region is ‘U’-shaped in a longitudinal direction of the active region.
 26. The method according to claim 25, wherein the active region defined by the second trench is etched so that the height of the ‘U’-shaped portion ranges from about 100 to about 2,000 Å.
 27. A method for manufacturing a semiconductor device, the method comprising the steps of: forming a first trench having a negative slope in a semiconductor substrate to define an active; forming a hard mask barrier layer over the semiconductor substrate; etching the lower part of the first trench by using the hard mask barrier layer as an etching mask to form a second trench having a positive slope; removing the hard mask barrier layer; forming a device isolation film to fill the first and second trenches; etching a give the region of the active region to form a first recess region; isotropic-etching the lower part of the first recess region to form a second recess region and a bulb-type recess region including the first and second recess regions; and forming a gate over the semiconductor substrate including the bulb-type recess region.
 28. The method according to claim 27, wherein the sidewalls of the first trench formed with the negative slope become wider downward based on a device separating region.
 29. The method according to claim 27, wherein the sidewalls of the second trench formed with the positive slope become narrower downward.
 30. The method according to claim 27, wherein the first trench has a depth ranging from about 1,000 to about 5,000 Å.
 31. The method according to claim 27, wherein the hard mask barrier layer is selected from the group consisting of a carbon layer, an oxide silicon nitride film and combinations thereof, with its thickness ranging from about 10 to about 500 Å.
 32. The method according to claim 27, wherein the second trench has a depth ranging from about 1,000 to about 5,000 Å.
 33. The method according to claim 27, further comprising forming a sidewall barrier layer on sidewalls of the first and second trenches.
 34. The method according to claim 33, wherein the sidewall barrier layer is selected from the group consisting of an oxide film, a nitride film and combinations thereof, with its thickness ranging from about 20 to about 200 Å.
 35. The method according to claim 27, wherein the first recess region has a depth ranging from about 1,000 to about 5,000 Å.
 36. The method according to claim 27, wherein the second recess region has a depth ranging from about 100 to about 2,000 Å.
 37. The method according to claim 27, wherein the first recess region has the active region whose center defined by the second trench is etched at a given depth, the second recess region is ‘U’ shaped in a longitudinal direction of the active region.
 38. The method according to claim 37, wherein the depth of the etched active region for the second recess region ranges from about 100 to about 2,000 Å. 